Objectives: to study immunologically induced tumors from the points of view of etiology, pathogenesis, characteristics, prevention and modification. The models used will be the graft-versus-host reaction (GVHR), post-thymectomy state (Tx), host-versus-graft reaction (HVGR), immunological tolerance, and graft-versus-graft reaction (GVGR). Methods: GVHR tumor induction, as an acute phenomenon (less than 50 days) will be emphasized. Evaluation of factors resulting in maximal tumor incidence in the SJL/J yields (SJL/J x C57BL/l)F1 model will be studied: host age, spleen cell donor age, route of donor cell injection, and especially, spacing of donor cell injections. Other studies include serial pathology, detection of donor cells in F1 recipients, donor and host contribution to splenomegaly, and effect of specific antiviral inhibitors, streptovaricin and poly IC. Combinations of other strains prone to lymphomas (AKR, SJL/J, C57BL/l) will be studied for acute tumor induction. Tx (SJL/J x C57BL/l)F1 mice will be studied for spontaneous tumor incidence and pathology and Tx (C57BL/l x A)F1 mice for an oncogenic agent in spontaneous tumors and in premalignant (autoimmune) lymphoid tissue. Mice suffering from HVGR, tolerant state and GVGR will be studied for tumor induction, tumor phenotype and viral etiology of tumors.